The invention relates generally to power amplifiers, and in particular to minimizing distortion in the input/output transfer function for amplifiers, particularly high power class AB power amplifiers.
High power, broad band power amplifiers are well known. These amplifiers may operate in a feed forward configuration, or may have other forms of linearization which are required when the main power amplifier operates, for example, as a class AB amplifier. Although class A amplifiers usually produce less distortion than class AB amplifiers, class A amplifiers are also less efficient than class AB amplifiers. Thus, in order to retain the advantages of efficiency while minimizing distortion, class AB amplifier configurations have been developed which implement various forms of error or distortion correction.
For example, one form of error correction attempts to distort the input signal in a manner which compensates for the distortions of the class AB amplifier. Thus, a predistortion circuit can be provided with various manual adjustments to produce a distortion signal from the original signal, so that when the distortion signal is combined with the input signal, and the combination is input to the power amplifier, operating for example, as a class AB amplifier, the output is substantially a linear amplification of the original input signal to the amplifier arrangement.
Such predistortion circuities typically employ a low power amplifier, preferably having the same general distortion characteristics as the main amplifier, so that its output, properly processed, can be used to obtain the necessary distortion components required to be combined with the input signal to the predistortion circuitry to generate a predistorted input to the main amplifier. Such configurations operate to substantially reduce the intermodulation frequency distortions produced by a class AB amplifier when the variable elements of the predistortion circuitry are properly adjusted.
However, even in properly adjusted amplifier arrangements using predistortion, a certain amount of instability can be observed. For many amplifiers operating in a Class AB mode, the bias point of the amplifier can affect the distortion it produces. Further, in current lateral MOSFET's operating as a class AB amplifier, amplifier component drift can be caused by temperature transients, and if the amplifier characteristics are sensitive to average power input, there can be further amplifier instability. Typically, however, these parameters are relatively slowly varying (for example a typical response time would be on the order of one millisecond); and even a low bandwidth amplifier has a signal bandwidth of one megahertz which corresponds to a period of one microsecond. While these instabilities have been attended to in a feed forward cancellation loop circuitry, for example, they are not adequately handled at the power amplifier itself.
Thus, the invention provides an advantageous approach toward maintaining stability in the input/output relationship of a high power class AB power amplifier arrangement. There results better linear stability over time and lower intermodulation distortions in the amplifier output.